Reflective mode light valves may be generally characterized as photovariable voltage dividers. A layer of fixed resistance liquid crystal material and a layer of variable resistance/photosensitive material form the elements of the voltage division network. Relatively low intensity writing (or image) light, when applied to the photosensitive element, produces a spatially resolved decrease in the resistance thereof with a corresponding, spatially resolved increase in the amount of current (measured r.m.s. in the case of an a.c. activated light valve) through the liquid crystal material. The increased current through the fixed resistance of the liquid crystal material produces a spatial voltage profile therein which creates a corresponding spatially resolved rearrangement of the optical properties of the layer of liquid crystal. The physical reorientation of optical properties is, of course, a function, inter alia, of the physical characterization of the liquid crystal material and the mode of activation (a.c. or d.c.) employed. The writing light and the beam of projection light do not interact optically but are coupled solely via the voltage division/liquid crystal reorientation process. As a result of the spatial recharacterization of the optical properties of the liquid crystal material, the relatively high intensity beam of projection light is modulated upon reflection from a reflective surface (often a dielectric mirror) underlying the layer of liquid crystal material. Thus the image is projected from the opposite surface of the light valve at a substantially enhanced intensity.
Reflective mode light valves operating according to the above-referenced principles may be either a.c. or d.c. activated. The basic principles of the a.c. activated light valve are disclosed in U.S. Pat. No. 3,824,002 entitled "Alternating Current Liquid Crystal Light Valve" issued to Beard and assigned to the assignee herein. In general, a.c. activation, where appropriate for the specific liquid crystal material employed, increases the chemical stability of the resultant device although various modes of operation, including the field effect and dynamic scattering modes, may be achieved by a variety of liquid crystal materials (including nematic and cholesteric) activated by either a.c. or d.c.
A significant variation in the characteristics and operation of light valves according to the reflective mode may be traced to the material employed as the photovariable resistance (photosensitive) layer of the device. For the near infrared band, intrinsic silicon, a well-known semiconductor material, provides a very attractive photoactivated variable resistance. A rigorous theoretical exposition of the various processes by which an a.c. activated light valve utilizing a semiconductive substrate of intrinsic material produces a high intensity image is described in the commonly assigned copending U.S. Pat. No. 4,191,454 for "Continuous Silicon MOS A.C. Light Valve Substrate" by P. O. Braatz, et al. As disclosed in that application, the pulsed a.c. bias cycle alternates periods of (potential) liquid crystal excitation (depletion of the substrate of majority carriers) with periods of relaxation (minority/carrier-recombination).
Regardless of the nature of the bias applied, it is essential in both a.c. and d.c. modes to apply the bias voltage across the combination of fixed resistance liquid crystal material and the variable resistance/photosensitive element. One equipotential surface commonly utilized for the application of such bias consists of a transparent, electrically conductive counterelectrode layer affixed to the inner surface of a glass plate adjacent the layer of liquid crystal material.
In a light valve employing a semiconductive substrate, an unavoidable amount of current due to thermally generated minority carriers (denominated "dark current") exists throughout the light valve regardless of the presence or absence of writing light. In a properly functioning light valve, the effects of this current are minimal and the resultant voltage across the liquid crystal material is generally well below the threshold required for significant optical effect. Thus, the minority carriers generated by the photons of the incident writing light, when present, predominate to produce the desired electro-optic effects. However, it has been found that, at biases in the range of 10-30 volts, the phenomenon of dark current breakdown is observed. That is, a significant increase in the sensitivity of minority carrier generation to an incremental increase in voltage is observed in this range, resulting in significant dark current through and a concurrent significant (above the electro-optical threshold) voltage drop across the liquid crystal material. The effect of such a breakdown is the obliteration of the photosensitivity of the device, preventing the projection of image in the affected areas and significantly limiting the operational range of the light valve.